Concurrent connections

[Arkadi Colson]

Is there a limitation on the number concurrent connections to a Solr 
host? Because we have some scripts running simultaious to fill Solr and 
when starting up to many we are getting this error:

exception 'SolrClientException' with message 'Unsuccessful update 
request. Response Code 0. (null)' in solr_queue_processor.php:467
Stack trace:
#0 solr_queue_processor.php(467): 
SolrClient->addDocument(Object(SolrInputDocument))
#1 {main}

Thx